The electrolytic industry typified by chloroalkali electrolysis has played an important role in the material industries. Although chloroalkali electrolysis has such an important role, the energy consumed for chloroalkali electrolysis is large and thus in countries, such as Japan, where the cost of energy is high, energy saving becomes an important consideration. For example, in chloroalkali electrolysis, to solve environmental problems and to attain energy saving, the electrolysis method is converted from a mercury method to an ion-exchange membrane method through a diaphragm, whereby energy saving of about 40% has been attained over about 25 years. However, even these energy savings need improvement, and the electric power cost i.e., the cost of energy, makes up 50% of the total production cost in the chloroalkali electrolysis. Unfortunately, as long as the present method is used, it is impossible to save more electric power. In other words, to attain more energy saving, a radical change such as the correction of the electrode reaction, etc., must be accomplished.
A gas-diffusion electrode having a property of easily supplying a gas as a reaction material to an electrode surface, was developed based on the uses such as fuel cells, etc., as described in H. Wendt, Electrochemical Hydrogen Technologies, pages 373-483, 1990 and E. A. Ticianelli at al., Journal of Electroanalytical Chemistry, 251, pages 275-281, 1988. Recently, the use of gas-diffusion electrodes for an industrial electrolysis has been investigated. For example, in an on-site production apparatus of hydrogen peroxide, a hydrophobic cathode for performing an oxygen reduction reaction is utilized as described in D. Pletcher, Industrial Electrochemistry (2nd edition), pages 279-282, 1991. Also, in an alkali production process or an alkali recovery process, in place of oxygen generation at the anode or hydrogen generation at the cathode as a counter electrode reaction, the hydrogen oxidation reaction (J. Jorissen et al, Journal of Electrochemistry, 21, pages 869-876, 1991) or the oxygen reduction reaction at a cathode (Miura, et al., Journal of Chemical Society of Japan, pages 732-736, 1982) is performed using a gas-diffusion electrode to effect reduction of electric power consumption. Also, it is reported that depolarization by a hydrogen anode as the counter electrode for metal recovery such as a zinc recovery, etc., and zinc plating is possible (Furuya et al., Denki Kaaaku Oyobi Kogyo Butsuri Kaaaku (Electrochemistry and Industrial Physical Chemistry), 56, pages 653-655, 1988).
However, in these industrial electrolyses, problems arise from the compositions of solutions and gases or the operation conditions, etc., being far more complicated than those of fuel cells. Additionally, sufficient life and performance of electrodes are not obtained.
As described above, in chloroalkali electrolysis, energy saving is advanced and the electric power source amount w for producing a caustic alkali is reduced to 2,000 kWh per ton of the caustic alkali. However, if using an oxygen cathode reaction in place of the conventional hydrogen generation reaction, the theoretical electrolytic voltage can be reduced from 2.19 V in the conventional method to 1.23 V, then large energy saving accompanied by the reduction can be expected.
For realizing the new process, the development of an oxygen cathode demonstrating high performance and sufficient stability in the above-described electrolytic system is indispensable (Fujita, et al., Dai 8 kai, Soda Kogyo Gijyutu Tooronkai Yousi Shu (The 8th Soda Industrial Technique Forum Summaries), 1984; Furuya, The 11th Soda Industrial Technique Forum Summaries, 1987; and Aikawa, Soda to Enso (Soda and Chlorine), 45. page 45, 1994).
In a salt water electrolysis using a conventional gas diffusion cathode, for example, an electrolytic cell partitioned by an ion-exchange membrane into an anode chamber and the cathode chamber, the cathode chamber is partitioned by the above-described gas-diffusion cathode into to sections to form a solution chamber at the ion-exchange membrane side and a gas chamber at the opposite side thereof. In this method, an oxygen-containing gas is introduced into the gas chamber and a caustic alkali is produced in the solution chamber.
The gas-diffusion cathode used for the electrolysis must be a so-called gas-liquid separation-type gas-diffusion cathode which sufficiently permeates oxygen only and does not leak caustic alkali from the solution chamber to the gas chamber.
The gas-diffusion cathode (oxygen cathode) proposed at present as the electrode for a salt water electrolysis meeting the above-described requirement is produced by applying a catalyst such as silver, platinum, etc., onto an electrode base material formed in a sheet form with a mixture of a carbon powder and PTFE.
The anodic reaction and the cathodic reaction in a conventional electrolytic method are EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e (1.36 V) EQU 2H.sub.2 O+2e.fwdarw.2OH+H.sub.2 (- 0.83 V)
respectively and the theoretical decomposition voltage is 2.19 V.
On the other hand, when the electrolysis is carried out while supplying an oxygen-containing gas to the cathode, the reaction becomes EQU 2H.sub.2 O+O2+4e V.fwdarw.4OH.sup.- ( 0.40 V),
and thus the electric power consumption can be reduced corresponding to theoretically 1.23 V and also to about 0.8 V even in the practical current density range, which means the reduction of 700 kWh per ton of a caustic alkali.
However, while a gas-diffusion cathode of this type can attain such a reduction of electric power, gas-diffusion cathodes present the following various problems.
(1) Carbon used as the electrode material deteriorates at high temperature in the presence of caustic alkali and oxygen, thereby greatly compromising electrode performance.
(2) It is difficult to prevent the leakage of caustic alkali generated into the gas chamber side, especially, when accompanied by the increase of the liquid pressure and the deterioration of the electrode associated with the present electrode as described above.
(3) The preparation of an electrode having the necessary size (1 m.sup.2) in a practical cell is difficult.
(4) The pressure in the cell changes according to the height and it is difficult to adjust the supplied oxygen gas pressure to cope with the change.
(5) There is an electrode resistance loss of the catholyte and also a motive power for stirring the catholyte is required.
(6) For practical use, it is necessary to greatly improve the existing electrolytic equipment.
(7) When air is used as the oxygen-containing gas, the carbon dioxide gas in air reacts with the caustic alkali to form sodium carbonate, which precipitates in pores of the gas-diffusion cathode, thereby lowering the gas diffusing capacity of the cathode.
To solve the various problems of the electrolytic method described above using the gas-diffusion cathode, a so-called zero-gap-type electrolytic method shown in FIG. 1 of the w accompanying drawings is proposed (Shimamune, et al., The 18th Soda Industrial Technique Forum Summaries).
An electrolytic cell 1 shown in FIG. 1 is partitioned by an ion-exchange membrane 2 into an anode chamber 3 and a cathode chamber (gas chamber) 4, a mesh-form insoluble anode 5 is closely contacted with the anode chamber 3 side of the ion-exchange membrane 3 and a gas-diffusion cathode 6 is closely contacted with the cathode chamber 4 side of the ion-exchange membrane 2. A mesh-form current collector 7 is in contact with the surface of the gas-diffusion cathode 6 and an electric current is supplied though the collector 7.
In addition, numeral 8 is an inlet for anolyte formed at the bottom plate of the anode chamber, 9 is a outlet for anolyte and gas formed at the upper plate of the anode chamber, 10 is an inlet for oxygen-containing gas formed at the upper plate of the cathode chamber, and 11 is an outlet for caustic soda formed at the bottom plate of the cathode chamber.
When while supplying, for example, an aqueous sodium chloride solution to the anode chamber 3 of the electrolytic cell and supplying an oxygen-containing gas to the cathode chamber 4, an electric current is passed through both the electrodes 5 and 6, caustic soda (sodium hydroxide) is formed at the surface of the cathode chamber 4 side of the ion-exchange membrane 2, the caustic soda reaches the surface of the cathode chamber side as an aqueous solution thereof through the gas-diffusion cathode 6 and the soda is recovered as a product.
The properties required for the gas-diffusion cathode (oxygen cathode) used for the above-described electrolytic method differ greatly from those of a conventional-type gas-diffusion cathode. The gas-diffusion cathode must have a sufficient hydrophobic property to avoid being wetted by the aqueous caustic alkali solution. The gas-diffusion cathode must also have adequate liquid permeability, etc., so that the aqueous caustic soda solution can easily permeate through the inside of the cathode. For these purposes, the electrode base material is constructed with a durable metal such as nickel, silver, etc., whereby the above-described problem (1) is solved and electrolysis of a long duration of time can be expected.
Also, because the above-described electrolytic method incorporates a system for recovering the caustic alkali that permeates to the back surface of the electrode, a partition between the solution chamber and the gas chamber in accordance with the above-described gas-diffusion cathode is unnecessary. Accordingly, the electrode is not required to be an integrated structure. Therefore electrode production can be scaled up relatively easily, thereby solving problem (3) discussed above.
Furthermore, as a matter of course, the liquid pressure change due to height variation of the liquid does not occur. Therefore the above-described problem (4) is solved. Also, because the aqueous caustic alkali solution formed in the process reaches inevitably the back surface of the electrode through the inside of the electrode, the above described problem (7) is minimized.
As described above, regarding the salt water electrolytic method, various improvements are made so that the method becomes suitable for an industrial electrolytic system as shown in FIG. 1. In contrast, when the existing electrolytic cell having a height of about 1 meter is utilized, the improved electrolytic method is still insufficient and tends to provide inadequate electrolytic performance. It is assumed that in addition to the aqueous caustic alkali solution formed moving to the back surface of the electrode, a part of the aqueous caustic alkali solution that is formed moves in the direction of gravity along the height of the cell (i.e., descends to the height direction or the direction of gravity). It is further assumed that when the amount of the caustic alkali solution formed exceeds the capacity of the gas-diffusion cathode itself to remove the caustic alkali solution moving in the height direction, the solution is retained in the inside of the electrode, whereby the supply of an oxygen-containing gas and the discharge of the produced gas are obstructed resulting in lower electrolysis performance.